A seat belt is used as a safety device which protects an occupant during a vehicle collision, and a force limiter of the seat belt is known as an accessory device for improving a performance of protecting the occupant. The force limiter gradually loosens a shoulder belt which is obliquely hung from a shoulder of the occupant to a waist, so as to alleviate an impact which is applied to the upper body of the occupant restricted by the shoulder belt. Typically, the force limiter is provided in a retractor which winds the seat belt.
In addition, similarly to the above-described force limiter, a vehicle seat is known which is configured to alleviate an impact, which is applied to an upper body of the occupant restricted by the shoulder belt, by a plastic deformation of a frame material of the seat (for example, see Patent Document 1).
The vehicle seat described in Patent Document 1 is a so-called belt-in seat in which the retractor which winds the seat belt is embedded. A frame of a seat back is divided into a lower structure and an upper structure which is supported by the lower structure to be tiltable to a front side of a seat, and an upper end of the shoulder belt is moored in a side frame of one side of the upper structure. Further, a torsion bar is bridged between the side frame of one side of the upper structure which moors the shoulder belt and an opposite side frame of the lower structure. When a tensile force exceeding a predetermined value is loaded on the shoulder belt, the upper structure is tilted to the front side of the seat in accordance with a torsion of the torsion bar such that the impact is alleviated.